Local Area Network (LAN) mirroring is used in systems which require a fault tolerant communication mechanism. In general, LAN mirroring means that frames sent between a sender and a receiver are sent via multiple routes (two or more). The basic idea behind LAN mirroring is that it is highly unlikely that all the duplicates of a frame would disappear. Therefore, the receiver will receive at least one of the frames. Usually the redundant LAN architecture assumes that a failure in one LAN is usually isolated from another LAN(s). Thus a node can communicate with other nodes even if one LAN cannot be used to send frames to other nodes.
FIG. 1 describes a basic system architecture for LAN mirroring. FIG. 1 comprises a sender, a receiver and two distinct LANs LAN1, LAN2 which connects the sender and receiver to each other. The sender sends a frame #X to via both LANs to the receiver. Although the main idea behind using LAN mirroring is very simple, there still exists some difficulties that must be taken care of:                The receiver must discard the duplicates, i.e. the receiver must know which frames it must accept and which reject.        The receiver must get back in synchronization after a failure of a sender or a receiver without requiring assistance from the user or having to inform the sender.        The sender and receiver must get in synchronization after the system has been started or restarted.        LAN mirroring must be transparent to upper level protocols (e.g. Internet Protocol (IP) and Address Resolution Protocol (ARP)).        